1. Field of the Invention
The present invention pertains to an ultraviolet laser apparatus as a light source for an exposure device and for an aberration measurement interferometer.
2. Description of the Related Art
Semiconductor fabricators are motivated to increase semiconductor performance and reduce manufacturing costs, factors that are greatly affected by the semiconductor fabrication process. Optical lithography is a preferred fabrication process for producing semiconductors. An optical lithographic system includes an exposure device, a mask, and resist.
Semiconductor performance is affected by the size of circuits on the semiconductor substrate. Thus, a goal in semiconductor fabrication is to create a small feature size, that is, a minimum size of an object formed semiconductor substrate. The minimum feature size that can be fabricated by an optical lithographic process is proportional to the wavelength of a light source of the exposure device. Previous optical lithographic systems used light from a mercury lamp having a wavelength of 436 to 365 nm. Currently, there is much research to optimize optical lithographic systems that have a laser light source with wavelengths less than 250 nm in order to create a smaller minimum feature size on semiconductors.
Additionally, projection optical systems in exposure devices must meet exacting standards, including low aberration. Such optical system must be evaluated to measure the degree of aberration. Satisfactory evaluation requires that the means for evaluating the optical system use the same wavelength as the light source of the exposure device.
A light source that can produce short wavelength light energy is an ArF excimer laser, which outputs laser light with a wavelength of 193 nm. However, ArF lasers produce light that has undesirably low coherence. Also, ArF excimer lasers are switched lasers that produce pulsed light energy that undesirably damages sensitive optical elements with its strong pulse peak power.
One method, other than ArF lasers, that produces short wavelength light is sum frequency generation (SFG). This method sums the frequencies of two light sources to produce an output light that is of a substantially higher frequency. Because wavelength is inversely proportional to frequency, SFG increases the frequency and decreases the wavelength of light.
A conventional method of performing sum frequency generation uses a nonlinear optical crystal and pulsed laser light from two lasers. By using pulsed laser light, the nonlinear optical crystal does not have to be located in a resonator. Good results may be obtained if laser lights of two wavelengths are made to simultaneously pass through the crystal. However, it has been difficult to achieve long coherence length when using pulsed light as the light source.
Another SFG method discloses first and second lasers and a nonlinear optical crystal located in a first laser resonator. Light from the second laser passes through the crystal only once and does not resonate. Because the second laser light does not resonate, its intensity is weak at the site of the nonlinear optical crystal, resulting in weak output intensity of the SFG light.
Another disclosure proposes an apparatus in which a nonlinear optical crystal is placed in an external resonator which resonates a first laser (a Ti:sapphire laser generating wavelengths from 750 nm to 810 nm). Light from a second laser generating 257 nm wavelength light (frequency doubled argon gas laser with a fundamental wavelength of 515 nm) passes through the crystal once and generates 194 nm light. The second laser light is not resonated, so its intensity is weak at the site of the nonlinear optical crystal, and as a result the intensity of the generated SFG light is about 4 .mu.W; not suitable for optical lithography.
Yet another proposed method simultaneously introduces two continuous laser lights into one external resonator and controls the lasers so they simultaneously resonate at some wavelength and perform sum frequency generation. This method requires one servo-controlled apparatus to synchronize the external resonator for the first laser, and a second servo-control apparatus to synchronize the wavelength of the second laser with the external resonator. The two servo-controlled apparatuses have the disadvantage of making the structure complicated, and may lead to system instability. Another defect is that coupling efficiency is bad when light from the first and second laser is incident upon the external resonator, efficiency falls and the output power diminishes.